Conventionally, a plurality of types of analog-to-digital converters, which differ in operation system, has been known. Such an operation system includes what is called a counter-ramp system and a successive approximation system. In the counter-ramp system, a ramp voltage whose voltage value changes with a predetermined slope, and an analog signal are compared with each other by a comparator, and a digital value of the analog signal is obtained from the number of counts performed until the two voltages match each other. In the successive approximation system, a referential voltage generated by a combination of a plurality of capacitances is made to gradually approach an analog signal while the referential voltage and an analog signal are compared with each other by a comparator, and an output of the comparator at that time is obtained as a digital value of the analog signal.
An analog-to-digital converter, which digitally converts an analog signal in two steps by dividing the analog signal into higher-order bits and lower-order bits, has been known (see, e.g., Patent Literature 1).